The present invention concerns a process for localizing a mobile unit on a network, and especially a process that makes it possible to localize with high accuracy the position of a mobile unit on a system of railroads, as well as a system for implementing such a process.
In order to localize a mobile vehicle on a railroad track, a known process consists of arranging detectors at specific points or sectors on the network, for instance electrical, electromagnetic or electronic circuits, placed on the track, and pedals that are probably associated as axle counters cooperating with the vehicle, etc.
However, such equipment which is linked to the infrastructure is costly, especially because of the high number of detectors required, and the investment often seems unproductive on low traffic density lines. Furthermore, knowledge of the position of the mobile unit is accurate only when it is in well determined areas on the line. In the areas between the well determined areas on the line, knowledge of localization is inaccurate and can become a hamper to the development of specific applications. Finally, knowledge of localization is locally available on the ground, but not directly on the mobile unit proper. In some instances, the detector tells the mobile unit that it is crossing at its level and sends it instructions pertaining, for instance, to speed limit or stopping distance. In those cases, for controlling speed or automatic piloting, the mobile unit can implement a measurement of the distance traveled, but it is not localization as such. Indeed, aside from the moment when the mobile unit crosses above the detector, the ground based system does not know exactly where the mobile unit is; whereas the mobile unit knows the distance traveled from the detector, but does not know its position in relation to the network as a whole. More importantly, the mobile unit it does not know which track it is on, within a network where several tracks might be contiguous.
Another known process consists of using a triangulation method with beacons, that can be static or mobile, and for instance loaded on board satellites placed in orbit.
This second process displays various inconveniences, aside from its high cost. Indeed, the available accuracy does not make it possible to distinguish, for certain, on which track of a network the mobile unit is located. Localization requires the possibility of a link, by radio usually, with the beacons, and irregularities in infrastructure relief, especially tunnels, do not make it possible to obtain good coverage of the network. Finally, at least in the case of radio-spotting, in other words when the ground based system acquires the knowledge of localization of the mobile unit, the ltter only obtains periodic knowledge of its position; it has to resort to estimation for constant monitoring of the speed or automatic piloting.
The patent FR 2 139 471 describes a control system for a vehicle that moves on a track divided into consecutive sections, which includes information on the distance traveled, thus making it possible to identify each section when the vehicle passes by. The patent FR 2 274 094 describes a device for localizing vehicles on a road network aimed especially at detecting changes in direction at crossroads.